How Cows Make Milk

The following topics have been designed to educate students about how cows make milk:

How Cows Make Milk

A cow only starts to produce milk once her first calf is born.

Cows typically have their first calf when they are only two years old. A cow is pregnant with her calf for nine months.

The usual practice is that a cow becomes pregnant again about 100 days after her calf is born.

Once she is pregnant she continues to give milk for about seven months. The farmer stops milking her two months prior to the birth so she can devote all her energy to producing her new calf.

A cow usually produces milk for as long as she is milked.

Cows belong to a group of animals called ruminants. These animals have four stomach compartments, each of which plays a different role in digesting food. Other ruminants include goats, sheep, giraffes and camels.

It takes 50 to 70 hours for a cow to turn grass into milk and most cows give about 25 litres of milk a day.

For every litre of milk the cow makes, more than 400 litres of blood must travel around the udder to deliver the nutrients and water for making milk. A cow has about 45 litres of blood in her body, so her blood is always on the move around the udder to keep making milk.

Before a cow can start producing milk, she must have delivered a calf. Cows usually produce a calf each year.

To produce milk, cows must eat a variety of grasses, clover and bulky fodder, which make them feel full, plus food rich in protein and energy.


The four stomach compartments and their special functions are:

1. The rumen

When cows graze on grass they swallow it half-chewed and mix it with water in their first stomach - the rumen - which can hold about 100 litres of chewed grass. It is here that the digestive process begins. The rumen softens and breaks down the grass with stomach juices and microbes.

2. The reticulum

In the reticulum the grass is made even softer and is formed into small wads called cuds. Each cud is then returned to the mouth where the cow chews it 40 to 60 times (for about one minute).

3. The omasum

The chewed cud is swallowed into the third stomach, the omasum, where it is pressed to remove water and broken down further.

4. The abomasum

The grass then passes to the fourth stomach, called the abomasum, where it is digested. The digested grass then passes through the small intestine, where all the essential nutrients the cow needs to stay healthy and strong are absorbed, and some are transported to the udder.



The nutrients from the grass are turned into milk by four mammary glands found in the udder. The droplets of milk are drained through an opening into the udder (called a duct) which is like a holding bag. The milk is released from the udder through the teat. Suction from a calf or the milking machine draws out the milk. The teat has a muscle called a sphincter which stops the milk dribbling out when the cow isn't being milked.

Milking Time

On most farms, dairy cows are milked twice a day in specially designed, electronically controlled milking sheds. Milking stalls in these sheds can be set out in a 'herringbone' pattern, or mounted on a continuously rotating platform, to allow for easy and efficient movement of cows with minimum need for individual handling. In most dairy sheds, cows are fed hay, grain or special mixed feed rations while they are being milked, which improves their health and ensures they receive the nutrients needed to produce high-quality milk. It also encourages them to come in to be milked.


                    Herringbone shed                                                      Rotary shed

Milking time is an enjoyable experience for the cows for many reasons. Sometimes the farmer plays soothing music in the background to relax the cows. It is in the farmer's best interest that the cows are kept happy because they need to be relaxed to produce their milk.

Cows will wait at the gate that leads to the milking shed so that they are first in line to be milked. Once the farmer opens the gate, the cows make their own way into the milking shed.

Occupational health and safety rules operate in all workplaces, including dairy farms. Most milking sheds are designed so that the cows stand on a raised platform for milking. This protects the farmer's back because he or she does not have to bend over to reach the cows' udders.

Once inside the shed, the cows stand beside each other and face in the same direction. The person milking is positioned behind the cows and moves from one cow to the next, working quickly and quietly to get the work done.

A suction cup on the end of a milking line is fitted onto each of the cow's four teats. The main milking machine uses a pump to create a vacuum, with a special controller that converts this into a series of 'vacuum surges' in each milking line. This cleverly sucks milk out of the teats in pulses, in much the same way as a calf does. The milk is drawn through the flexible milking lines into stainless steel pipes, and into a refrigerated storage tank called a vat, which cools the milk down to 4 degrees Celsius.

From the vat, the milk is collected by a milk tanker within just a few hours. Tankers have large, stainless steel refrigerated tanks that keep the milk clean and cold. Tankers have the capacity to carry up to 29,000 litres.

Before accepting the milk, tanker drivers test it for freshness and quality to ensure it complies with all health standards. This is just one of the checks carried out to ensure that consumers receive a quality product.

Robotic Milking

In Europe, over half of new milking machines involve automation (robotic milkers). This milking technology has great potential for improving labour use efficiency as the entire milking procedure occurs without human assistance. The cow voluntarily walks to the milking shed to present herself for milking where she is recognised by an electronic transponder. The suction cups are attached to the udder by a robotic arm and are removed after milking has finished.


Top: a robotic milking machine / Bottom: robotic arm attaching the suction cups.

The automatic milking system offers farmers a more flexible lifestyle as it will free up several hours a day previously spent milking cows, leaving farmers to concentrate on other farm and business management; for example monitoring the performance of individual cows and the whole farm system. It also gives them a sleep in as it means no more 5.30 am wake up calls for milking.

Cows voluntarily make their way to the milking shed, as they know they'll be rewarded with grains to eat while milking and fresh pasture to enjoy afterwards. They are often motivated by these factors to present themselves for milking more often, which means that high producing cows can be milked more often (every 6 to 12 hours) compared to lower producing cows (every 12 to 24 hours).

Automatic milking systems typically require major changes to other aspects of the farm system and 'The Future Dairy' project led by Dairy Australia is defining systems suitable for use in the Australian dairy industry.

There are already four Australian dairy farms using the automatic milking system with much success. Check out a farm using this technology by visiting

Now go to On the Farm.